So, you are missing several factors:

1. compression ratio means that the pressure is above atmospheric - probably what you assumed with your calculation. The pressure AND temperature is also increased - something used in diesel engines so they don't need sparks...

2. the spark needs to be "big & fat" ie just breaking down to have a feeble spark is not good enough,

3. the mechanics of the combustion process also come into play here, as the initial combustion from the spark has a delay time then the flame front will progress through the fuel charge, that is why there is spark advance so that the fuel charge has sufficient time to burn during the power stroke.

Champion used to make sparkplug testers, which had a chamber that the sparkplug to be tested was screwed into. There was a viewing port and a spark generator. The test involved watching how the spark behaved as one increased the air pressure (workshops have compressed air). As the pressure increased then if the insulation - the cone - was cracked or contaminated then the spark would deviate from just between the terminals. Good kit and solved many misfire issues...

So, you are missing several factors:

1. compression ratio means that the pressure is above atmospheric - probably what you assumed with your calculation. The pressure AND temperature is also increased - something used in diesel engines so they don't need sparks...

2. the spark needs to be "big & fat" ie just breaking down to have a feeble spark is not good enough,

3. the mechanics of the combustion process also come into play here, as the initial combustion from the spark has a delay time then the flame front will progress through the fuel charge, that is why there is spark advance so that the fuel charge has sufficient time to burn during the power stroke.

Champion used to make sparkplug testers, which had a chamber that the sparkplug to be tested was screwed into. There was a viewing port and a spark generator. The test involved watching how the spark behaved as one increased the air pressure (workshops have compressed air). As the pressure increased then if the insulation - the cone - was cracked or contaminated then the spark would deviate from just between the terminals. Good kit and solved many misfire issues...

Denso has some good information (should do they make sparkplugs...):

enter link description here

So, you are missing several factors:

1. compression ratio means that the pressure is above atmospheric - probably what you assumed with your calculation. The pressure AND temperature is also increased - something used in diesel engines so they don't need sparks...

2. the spark needs to be "big & fat" ie just breaking down to have a feeble spark is not good enough,

3. the mechanics of the combustion process also come into play here, as the initial combustion from the spark has a delay time then the flame front will progress through the fuel charge, that is why there is spark advance so that the fuel charge has sufficient time to burn during the power stroke.

So, you are missing several factors:

1. compression ratio means that the pressure is above atmospheric - probably what you assumed with your calculation. The pressure AND temperature is also increased - something used in diesel engines so they don't need sparks...

2. the spark needs to be "big & fat" ie just breaking down to have a feeble spark is not good enough,

3. the mechanics of the combustion process also come into play here, as the initial combustion from the spark has a delay time then the flame front will progress through the fuel charge, that is why there is spark advance so that the fuel charge has sufficient time to burn during the power stroke.

Champion used to make sparkplug testers, which had a chamber that the sparkplug to be tested was screwed into. There was a viewing port and a spark generator. The test involved watching how the spark behaved as one increased the air pressure (workshops have compressed air). As the pressure increased then if the insulation - the cone - was cracked or contaminated then the spark would deviate from just between the terminals. Good kit and solved many misfire issues...